421G

See how it has the fold up bed, like a Featherweight or 301? I guess you’d call this a long bed.

Here’s the nameplate.
Again, note the bobbin winder on the top of the machine. If you were REALLY paying attention, you'd see that there isn't the rectangular plug in the base. My best guess is ... since this is a free arm machine, it wouldn't be mounted in a cabinet. Hence, it would never be set up as a treadle.

Here is under the cam cover. Again, it would use all the same Top Hat cams that all the 400 series machines use.

The 421G is a free arm machine, like the 431G. From what I gather, the 421G and the 431G are similar, except the 431G can do a chain stitch, and the 421G can't. I think that this matches the similarity of the 401G and the 411G machines.

There’s a neat storage compartment in the bed of the machine.

The 421 (and 431) have different needle plates than the 401, 403, 404 and 411 machines. I've read that these are similar to the later Touch & Sew machines, although I can't say if they are interchangable. Perhaps Singer was trying out the design on these machines?
The screws that hold the needle plate in place are slit, and are actually springs.
This spring force is what holds the needle plate in place. Unfortunately, these
screws are fragile, and one or more of the pieces of the screw head can (and often
do) break off. The left hand screw is missing one of its head segments. Also note
how the slide plate is curved to match the shape of the free arm.

Here’s a top view with the bed folded down. Again note the top bobbin winder.